1. Field of the Invention
The present invention relates to a servo writer that writes servo signals used for tracking control of magnetic heads to magnetic tapes.
2. Description of the Related Art
In recent years, the recording density of magnetic tapes has increased. Due to this trend, it has been necessary for servo writers that write servo signals to magnetic tapes to be highly accurate. This is because this high level of accuracy is needed to ensure that a servo writer's magnetic head traces track on a magnetic tape precisely. In such a servo writer, servo write heads are required to be exchanged, depending on the servo pattern to be written to a magnetic tape or the type of magnetic tapes. When servo write heads are exchanged, a process of adjusting the head precisely is necessary. Currently, this process is complex and time-consuming.
To overcome this disadvantage, the applicant has proposed a servo writer with a turret on which multiple servo write heads are mounted (see U.S. Pat. No. 6,873,587). In this servo writer, in order to exchange the servo write heads, the turret simply needs to be turned. This servo writer makes it possible to adjust the head without the need for stopping the writer for a long time. This enables much more efficient recording of servo signals on magnetic tapes and, therefore decreases the overall cost.
As for technique for writing servo signals, a servo writer supplies a record current to the servo bands of a magnetic tape, thereby magnetizing them in one direction. As a result, servo signals are written to the servo bands. Specifically, as shown in FIG. 9, conventional servo signals SS″ are formed on non-magnetized servo bands SB″ by supplying record pulse currents PC to the non-magnetized servo bands SB″. The record pulse current PC″ is composed of zero currents and positive pulse currents, because it is aimed at avoiding the saturation of MR elements of a magnetic head. In this record pulse current PC″, a magnetic tape MT is not magnetized when the zero currents are supplied. In addition, when the positive pulse currents are supplied, the magnetic tape MT is magnetized in one direction by means of leakage flux from servo gaps of the magnetic head. Finally, servo patterns SP″ are formed so that servo signals SS″ are written.
A magnetic tape recording/reproducing device has a servo signal reading element (MR element), and senses the variation in the magnetic flux generated from the servo signal SS″, based on the variation in the electric resistance of the servo signal reading element. Furthermore, the magnetic tape recording/reproducing device outputs, as a reading signal, a differential (voltage) waveform in proportion to the variation in the magnetic flux. Therefore as the variation in the electric-resistor of the MR element is larger, the peak-to-peak value of the reading signal generated from the servo signal SS″ increases. In other words, the SN ratio of the reading signal is improved. Further, when the magnetic flux from the servo signal SS″ is large, or when the servo signal reading element (MR element) is wide, that is, has a large sensing area, the peak-to-peak value of the reading signal RSL from the servo signal SS increases, as shown in this figure.
In the future, it can be expected that the recording density of magnetic tapes will develop to several tens of terabytes. Accordingly, the number of the data tracks on a magnetic tape increases. In addition, each data track is narrowed, the interval between the adjacent data tracks is also narrowed, and the magnetic tape is thinned. Hence, the quantity of the magnetism which can be sensed from the servo signal decreases, and the variation in the magnetic flux from the servo signal SS″ which can be sensed by the servo signal reading element also decreases. Therefore, the peak-to-peal value of the reading signal RSS″ from the servo signal SS″ decreases, as shown in FIG. 9D, thereby deteriorating the SN ratio of the reading signal RSS″. This may be impossible for a magnetic tape recording/reproducing device to read the servo signal SS″ precisely, thus deteriorating the positional control of the magnetic head. This can be a problem. Disadvantageously, the above-described servo writer with a turret, which has been proposed by the applicant, has not yet come to overcome the problem involved by such high density recording of magnetic tapes.
Taking the above disadvantage into account, the present invention has been conceived. An object of the present invention is to provide a servo writer in which servo write heads are easy to exchange, and which writes a servo signal from which the reading signal of an improved SN ratio can be obtained.